Heat-treatment and temperature gradient furnaces are used in a number of production processes, such as for annealing of metals and ceramics, for annealing and oxidation processes for semiconductors, for “growing” of crystals, and for other heat-treatment material process applications. Temperature gradient furnaces commonly have a series of zones to control temperatures around the area of a workpiece to promote crystal growth or to promote particular mechanical or electrical properties within a material. Each material application process requires steep gradients in temperature and the holding of consistent temperatures over long periods of time with minimal variation to produce uniform properties within the material.
As an example, in an application to grow large-area single-crystals, the crystals commonly formed from a melt of crystalline material are placed in a refractory container or ampoule and heated within a furnace at a constant temperature until molten. The molten material is then cooled slowly at one end until crystallization starting from a single nucleation from the melt sets in. Currently, the cooling at the crystallization temperature must be done slowly and progressively along the length of the material, so that the crystal lattice can form entirely on the single nucleation so as to produce a single crystal. Optimum conditions for single crystal growth call for substantially steep and well-controlled temperature gradients that move relative to the material. Furnaces that produce fairly reliable temperature gradients are known, such as the furnace described in U.S. Pat. Nos. 4,518,351 and 4,423,516 issued to Robert H. Mellen, where the furnace uses a plurality of heating elements sandwiched between respective insulating layers to form temperature zones across the material workpiece. The temperature within each zone is set and maintained or ramped up to a prescribed temperature by powering the heating elements using feedback from thermal sensors within each zone to adjust power requirements and create temperature gradients through the zones of the furnace.
Temperature adjustments for each specific zone provide for a more accurate gradient in temperature within the area of the work. However, variations or ripple at set temperatures within the temperature gradient is one of a number of factors that affect the quality of the monocrystals or properties and uniformity of other heat-treated materials. It has been extremely difficult to produce large area, single-crystals or other heat-treated materials that have low compositional variation and homogeneous mechanical or electrical properties. A factor in producing the temperature gradient is related to loading of the heating elements, where unless a heating element is continually loaded, its response to control can be slow and erratic causing inconsistent temperature fluctuations and temperatures exceeding the prescribed temperature gradients. Another factor affecting composition and material properties is the problem of heat flowing radially from the axis of the temperature gradient within the furnace chamber. Such heat flow produces undesired temperature variations within planes perpendicular to the axis of the gradient. This can cause deleterious effects in crystal growth particularly along the interface of the crystal and ampoule where temperature variations may cause a concave shape with respect to the crystal. The concave interface shape may promote new grains to form causing growth to be inward towards the bulk of the crystal reducing compositional homogeneity. Additionally, current crystal growth process techniques and other heat-treatment material processes require lengthy periods to adjust gradients over small areas within the work region, where cooling of the heating elements may require hours. Additional time is also required to reduce the temperature of the furnace to move the workpiece to perform additional process steps or to remove the crystals or materials from the furnace.
Therefore, proper furnace design for a single temperature zone or multi-temperature zone furnace is a compromise between insulating capability and the ability to effectively change temperatures inside the furnace. A well-insulated furnace is capable of high temperature ramp rates in the positive direction, but in cooling, the same insulation that allows the furnace to heat up quickly, makes the furnace unable to cool quickly, specifically because of the increased insulation.
In contrast, a furnace that is designed to cool quickly will have little or no insulation, which allows the furnace to drop temperature quickly. However, in this furnace design it is difficult to ramp the temperature up quickly without significant increases in power. This additional power degrades the life of the heating elements causing the furnace to have a shorter operational life. Less insulation also dissipates significant amounts of heat into areas around the furnace requiring fans and air conditioning systems around the furnace units to maintain acceptable ambient temperatures. Alternately, a water cooling system may be used to dissipate the additional heat increasing both system and facility costs where plumbing and refrigeration to cool and pump the water through the furnace system is required.
Therefore, significant additional costs for cooling systems and facility infrastructures must be considered in using single temperature zone, multi-temperature zone or other furnace systems of the prior art. What is not known is a less costly and more effective cooling system for a well-insulated furnace that provides suitable temperature gradients, assists in maintaining minimal variations in temperature, and cools rapidly thereby increasing throughput in the production of heat-treated materials.